Psychrometrics

1. Psychrometrics

2. Psychrometrics • It involves determination of thermodynamic properties of gas-vapor mixtures. • The most common applications are associated with the air-water vapor system.

3. Properties of dry air • Composition of air

4. Properties of dry air • Molecular weight = 28.9645 • Gas constant for dry air = 287.055 m3Pa/kg.K • Specific volume V’a = • Specific heat (average) = 1.005 kJ/kg.K • Enthalpy = Ha = 1.005 (Ta-To) kJ/kg • Dry bulb temperature = temperature indicated by unmodified sensor

5. Properties of water vapor • Molecular weight = 18.01534 • Gas constant for dry air = 461.52 m3Pa/kg.K • Specific volume V’w = • Specific heat (average) = 1.88 kJ/kg.K • Enthalpy = Hw = 2501.4 + 1.88 (Ta-To) kJ/kg

6. Properties of air-vapor mixtures • Gibbs-Dalton Law pB = pa + pw wherepB = total pressure pa = partial pressure of dry air pw = partial pressure of water vapor

7. Dew-point temperature • The water vapor in the air will be saturated when air is at a temperature equal to the saturation temperature corresponding to the partial pressure exerted by the water vapor. • This temperature is called dew-point temperature.

8. Humidity ratio/ moisture content (W) • PV = mRT (ideal gas law) • P = Pw + Pa R = gas constant P = total pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air

9. Relative Humidity • Φ = xw/xw,s = Pw/Pws = w/ws • Function of T • Easy to measure and useful in some contexts, but often need to know temperature as well x = mole fraction P = pressure μ = degree of saturation W = humidity ratio

10. Humid heat • It is amount of heat (kJ) required to raise the temperature of 1 kg dry air plus water vapor present by 1 K. • Cs = 1.005 + 1.88W

11. Specific volume • It is the volume (m3) of 1 kg dry air plus water vapor in the air. V’m =

12. Wet bulb temperature pw = where pw = partial pressure of water vapor pB = total pressure = barometric pressure pwb = saturation pressure of water vapor at wet bulb temp. Ta = dry bulb temp. Tw = wet bulb temp.

13. Example • Find dew-point temperature, humidity ratio, humid volume, and relative humidity of air having a dry bulb temperature of 40C and a wet bulb temperature of 30C.

14. Adiabatic saturation process • Phenomenon of adiabatic saturation of air is applicable to convective drying of food materials. • Adiabatic condition • Well insulated chamber: no heat gain and loss • Air is allowed to contact a large surface area of water • Part of sensible heat of entering air is transformed into latent heat

15. Adiabatic saturation process • Process of evaporation water into the air results in saturation by converting part of sensible heat of the entering air into latent heat

17. Psychrometric chart

18. Psychrometric Chart • Need two quantities for a state point • Can get all other quantities from a state point • Can do all calculations without a chart • Often require iteration • Many “digital” psychrometric charts available • Can make your own

19. Temperature • Absolute Temperature • Dry-bulb temperature • Wet-bulb temperature • Dew-point temperature

28. Psychrometric Charts • Make sure chart is appropriate for your environment • Figure out what two quantities you know • Understand their slopes on the chart • Find the intersection • Watch for saturation

31. Psychrometric processes

34. Sensible heating

40. Example

46. Dehumidification by Cooling

47. Example